Grease pencil stroke wont show while the pen is down
I faced the same problem as well. It is quiet annoying. The solution I found is : 
Install the latest blender 2.8 version and try drawing over the 3D object. If the problem persists add a new stroke or grease pencil from object mode and draw on that new stroke/ grease pencil. Adding a new stroke/greasepencil worked for me.